As disclosed, for example, in JP, U, 62-19475, a conventional traveling hydraulic-motor drive circuit system, which is used in a construction machine such as a hydraulic excavator or the like, comprises a hydraulic motor whose output shaft is connected to a traveler such as, for example, a crawler or the like, for rotatively driving a vehicle body which is an inertia load, a hydraulic pump serving as a main hydraulic source, a reservoir, a pair of main lines through which the hydraulic motor is connected to the hydraulic pump and the reservoir, and a directional control valve of normal open type arranged in the pair of main lines for switching a direction of hydraulic fluid supplied to the hydraulic motor from the hydraulic pump. further, a counter balance valve is arranged in the pair of main lines between the directional control valve and the hydraulic motor.
The counter balance valve comprises a pair of check valves, a pressure control valve arranged in parallel to the check valves and normally biased to a neutral position under biasing forces of springs, the pressure control valve being switched from the neutral position to left- and right-hand operative positions against the biasing forces of the springs when the hydraulic fluid from the hydraulic pump acts as pilot pressure, and a pair of restrictors for regulating or adjusting the switching speed of the pressure control valve. The pressure control valve is provided with intermediate restriction regions at respective locations between the neutral position and the operative positions, whereby when the pressure control valve is returned to the neutral position from the operative positions, a restriction action is given to return fluid from the hydraulic motor to generate brake pressure in the main line on the return side.
In the hydraulic-motor drive circuit system constructed as described above, when the directional control valve is switched to the operative positions from the neutral position, the hydraulic fluid from the hydraulic pump is supplied to the hydraulic motor through the directional control valve, one of the main lines and the corresponding check valve to rotate the hydraulic motor together with the inertial load. At this time, pressure within the main line (drive pressure of the hydraulic motor) on the outlet side of the directional control valve acts as pilot pressure on the pressure control valve of the counter balance valve through one of the restrictors. The pressure control valve is switched from the neutral position to the operative positions by the pilot pressure. Thus, the return fluid from the hydraulic motor is discharged to the reservoir through the other main line.
On the other hand, when the vehicle reaches a downward path or the like under the condition that the hydraulic motor is rotated together with the inertia load and then the hydraulic motor is caused to rotate by the inertia load, pressure within the main line on the supply side is reduced by a pump action of the hydraulic motor so that the pressure control valve of the counter balance valve tends to be returned to the neutral position from the operative positions through one of the restriction regions. Accordingly, the return fluid from the hydraulic motor is confined within the main line on the return side at a location between the hydraulic motor and the counter balance valve. Thus, the pressure in the main line on the return side is raised to a setting pressure of an overload relief valve which is provided in the pair of main lines, and brake pressure is generated in the main line on the return side, thereby preventing the vehicle from scampering at the downward path or the like.
Further, when the directional control valve is returned to the neutral position in order to halt or stop the vehicle under the condition that the hydraulic motor is rotated together with the inertia load, the hydraulic motor tends to continue to be rotated by the inertia load. Also in this case, since the pressure control valve of the counter balance valve tends to be returned to the neutral position from the operative positions through the restriction region, the brake pressure is generated within the main line on the return side similarly to the above, to halt the vehicle.
In the prior art, the pair of restrictors are provided in the counter balance valve as described above, and the return speed of the pressure control valve to the neutral position is regulated by the setting of the restrictors. In the case, however, where the restrictors are so set as to expedite the return speed of the pressure control valve, the main line on the return side is abruptly cut off or isolated by the pressure control valve so that shock or impact increases. Thus, it is usual to set the return speed of the pressure control valve to one which is more or less slow, in order to relieve the shock. Accordingly, in setting the counter balance valve described above, when the directional control valve is returned to the neutral position in order to halt the vehicle under the condition that the hydraulic motor is rotated together with the inertia load, the pressure control valve of the counter balance valve is not immediately returned to the neutral position.
During a period within which the pressure control valve is returned to the neutral position, the hydraulic motor continues to be inertially rotated so that a part of the hydraulic fluid (return fluid) discharged to the main line on the return side passes through the pressure control valve which is located in the restriction region. A part of the hydraulic fluid passes through the directional control valve of center open type located at the neutral position, and is discharged to the reservoir.
On the other hand, at this time, supply of the hydraulic fluid from the hydraulic pump is cut off completely by the directional control valve. Accordingly, although the hydraulic fluid within the main line on the return side is partially returned to the main line on the supply side through the overload relief valve, the operational hydraulic fluid within the main line on the supply side is insufficient, at least by the amount of the return fluid which has been discharged through the pressure control valve and the directional control valve, so that negative pressure is liable to be generated in the main line on the supply side. Thus, there is a fear that cavitation will occur.
Furthermore, in the above-described operation, after the pressure control valve of the counter balance valve has been returned to the neutral position, all the hydraulic fluid, which has been discharged into the main line on the return side under the pump action of the hydraulic motor, is returned to the main line on the supply side through the overload relief valve. Thus, discharge of the hydraulic fluid to the reservoir through the pressure control valve cease. However, leakage of the hydraulic fluid inevitably occurs from movable parts of hydraulic instruments such as the hydraulic motor and the like, and also, during a period within which the vehicle is halted after the directional control valve has been returned to the neutral position, brake pressure occurs due to the overload relief valve at a location within the main line on the return side, so that an amount of leakage will particularly become noticeable or remarkable. Thus, there is a fear that the operational hydraulic fluid within the main line on the supply side will likewise be insufficient due to the internal leakage, so that the negative pressure is liable to be generated and cavitation occurs.
Occurrence of the above-described cavitation will generate relatively large noises. Not only does this give an unpleasant feeling to an operator, but it also damages the hydraulic instruments such as the hydraulic motor and the like, to lower the service life thereof. Thus, occurrence of the cavitation becomes an issue which cannot be ignored in view of the operation and an increase in durability.
Moreover, a traveling hydraulic-motor drive circuit system is known which is utilized in a small hydraulic excavator or the like, in which the counter balance valve is omitted, and a directional control valve of the closed center type is used in substitution for the directional control valve of the normally open type. In the drive circuit system, the directional control valve has intermediate restriction regions between the neutral position and the left- and right-hand operative positions. Accordingly, it is possible to use the restriction regions to limit the return fluid, and to relieve shock at stoppage of the hydraulic motor.
However, the circuit system has also the following problem. That is, a meter-in restrictor and a meter-out restrictor of the directional control valve are set such that the opening of the meter-out restrictor is larger than that of the meter-in restrictor, in order to enable speed control of the hydraulic motor by meter-in control. Thus, during a period within which the directional control valve is in the restriction regions, the hydraulic fluid discharged to the reservoir from the main line on the return side increases more than that supplied to the main line on the supply side, so that operational hydraulic fluid within the main line on the supply side will likewise be insufficient, which generates cavitation.
Particularly, after the directional control valve has been returned to the neutral position, the operational hydraulic fluid within the main line on the supply side will be sufficient due to internal leakage within the hydraulic instruments such as the hydraulic motor and the like, similarly to the above-described case provided with the counter balance valve. Thus, cavitation will occur.
The invention has been constructed in view of the above problems of the prior art, and an object of the invention is to provide a hydraulic-motor drive circuit system capable of effectively preventing cavitation from occurring during inertia rotation of the hydraulic motor, capable of enhancing service life of hydraulic instruments such as the hydraulic motor and the like, capable of reducing shock at stoppage of the hydraulic motor or the like, and capable of enhancing reliability.